Japanese patent laid-open publication No. 2014-17974 (“JP '974”) discloses a ground fault detecting device that can simultaneously carry out ground fault detection for a non-grounded circuit and detection of the supply voltage of the non-grounded circuit and can detect a ground fault irrespective of the ground fault potential based on a circuit configuration that allows achievement of size reduction. This ground fault detecting device includes a ground fault detecting unit and a supply voltage detecting unit. The ground fault detecting unit detects a ground fault of a non-grounded circuit based on a ground fault detection signal of a ground fault signal detecting circuit when a signal for detection is superimposed on a positive-side output unit and a negative-side output unit by an inverting amplifier connected to the positive-side output, unit via a first relay circuit and an inverting amplifier connected to the negative-side output unit via a second relay circuit. The supply voltage detecting unit detects the output voltage of a lithium ion battery based on a first supply voltage detection signal of a first supply voltage detecting circuit.